Long term investing or long term trading can mean different things to different types of investors or traders. If you are a day trader then a strategy that trades weekly will be long term. Some define long term as holding an asset for an extended period of time. The time could be as little as one year and as long as 30 years.
Advisers usually advise to invest for the long term but the long term is subjective. A buy and hold investor may consider long term 5 to 10 years. While a day trader may consider long term investing 5 to 10 days.